Minimising the impact of ballast water
Science Centres: Aquatic Biodiversity and Biosecurity
This map shows the risk factor for discharge in the 12-mile Territorial Sea. Click to enlarge.
New Zealand’s coastal areas support a disproportionately large number of endemic marine species, and there is little doubt that they could be affected by the arrival of exotic organisms. New exotic species could also threaten marine shellfish and aquaculture in areas such as Foveaux Strait, Marlborough Sounds, and Tasman and Golden Bays.
The high-profile arrival of the seaweed Undaria pinnatifida to New Zealand was probably due to ballast water. Because many species of algae and marine animals have remarkably long cyst or larval stages, they can arrive in a country as zooplankton, immature invertebrates, and even small fish. For example, the resting cysts of toxic dinoflagellates can be viable for more than 6 months, and the larvae of the Northern Pacific seastar can survive for up to about 150 days. The length of the larval stage has major impacts on whether or not organisms will be able to survive in the ballast water. Species with long cyst or larval stages are more likely to survive ocean crossings in ballast water, but there is still a chance that short-lived species will be able to survive the journey and adapt to a new environment.
Most marine invasions around the world occur in sheltered bays and estuarine environments, and almost all of New Zealand’s marine invasive species are confined to harbours or embayments. The large number of invasions into estuarine and harbour environments probably reflects the location of international ports and anchorages, and the similar environmental conditions between the source of the invasive species and the recipient ports.
The introduction of invasive marine species into new environments by ships’ ballast water or hulls (or other means) is of great concern to New Zealand. High-risk areas for deballasting probably occur where water is delivered rapidly to coastal environments, and where the residence time of the discharge in the coastal zone is long compared with the lifespan of larvae from any invasive species. This risk has two parts – the first is the physical likelihood that organisms from ballast water will remain in the coastal zone long enough to settle, and the second is the biological impacts of invasions on different coastal regions.
The main control on the dispersion of any alien species introduced into New Zealand coastal waters is the local coastal currents. (The ideal discharge site would have little commercial or cultural value, a low retention time, and predominately offshore winds.)
New Zealand sits in the eastward-flowing southern branch of the South Pacific gyre. This flow is sometimes called the Tasman Current, and is an extension of the East Australian Current. Near the west coast of the South Island the circulation is known as the Westland Current, and off the west coast of the North Island the flow is known as the West Auckland Current. Both are weak and variable in contrast to the western boundary current that causes the strong southwards-flowing East Auckland and East Cape Currents along the east coast of the North Island. The topography also causes a strong northwards flow along the east coast of the South Island known as the Southland Current. The other main currents are the D’Urville and Wairarapa Coastal Currents, which feed into the East Cape Current.
NIWA has assessed the risk of invasive species introduced in ballast water based on flow modelling of waters inside the 12-mile Territorial Sea. We have developed an index which rates the potential effect of marine discharge on the environment on a scale from 0 to 10, where 0 is no risk to the environment, and 10 is almost certain environmental impact if the discharge contains invasive species that can adapt to the local environment. The map does not consider the social and economic costs of invasive species.
Most of the northeast and west coasts of the North Island, and much of the South Island, has been rated 10 because there is a good chance that any discharge within the 12-mile Territorial Sea would remain in New Zealand’s coastal environment for days or weeks. The east coast of the South Island is only slightly less at risk from marine discharge than the highest-risk areas because there is still a chance that discharge along this coast will enter the bays and inlets around Banks Peninsula, Cook Strait, and the Marlborough Sounds.
The lowest-risk region (with a rating of 5) is the east coast of the North Island, where it is likely that the discharge will be taken away from the land by the East Cape Current. However, the east coast has high spiritual value for Maori, and is important to New Zealand’s rock lobster fishery as an area for puerulus settlement and recruitment.